Dental composite restorative material and method of restoring a tooth

ABSTRACT

A composite material is provided which, while having an unusually high filler content may be extruded from a dental syringe and remains easily adaptable in the dental cavity. When materials of the present invention are cured, dental restorations are provided which have unusually high surface hardness and yield strength, as well as a low volume shrinkage on curing. This is achieved by use of a mixture of filler particles with a specific size, size range, and size relationship. Such a combination of properties makes the material of the present invention particularly useful for restoring cavities in posterior teeth.

RELATED APPLICATION

[0001] This application is a continuation-in-part of pending applicationU.S. Ser. No. 09/978,741, which is a continuation-in-part of U.S. Ser.No. 09/632,065 now issued as U.S. Pat. No. 6,306,927.

BACKGROUND TO THE INVENTION

[0002] Dental composites, which essentially comprise a mixture of apolymerisable resin and a glassy filler, have been developing since theearly 1970's, when the first materials of this class were introduced.See for example R. L. Bowen et al., “A new series of X-ray-opaquereinforcing fillers for composite materials, J. Dental Research, vol.51(1) 1972. Until this time, fillings had been based on silver-mercuryamalgams, mixtures of acid leachable glass with phosphoric acid (knownas “silicate cements”), or unfilled polymerisable resins, and each classof material has certain strengths and weaknesses. For instance, amalgamsare generally considered to be cheap and easy to use, and to have a longlife time due to their strength and high resistance to wear.Disadvantages of amalgam are toxicity of the mercury and the blackcolour of the filling. Silicate fillings were approximately toothcoloured and released fluoride into the tooth to help prevent arecurrence of decay. However they tended to dissolve quickly and wereweak, and are barely used nowadays. Unfilled resins brought advantagesof toughness, convenience, and aesthetics, but were still weak, limitingtheir use to areas of low stress. These unfilled resins also have a highvolume shrinkage, commonly at least 5%. This leads to formation of gapsbetween the filling and the tooth, and subsequent recurring decay of thetooth around and underneath the restoration. The introduction ofcomposite materials brought improvements in surface hardness, higherphysical strengths, good aesthetics, lower shrinkage, and also higherresistance to wear. However the wear rate of these composite materialsis still higher than that of amalgam, and their shrinkage of around 2 to3 volume percent still leads to gap formation and recurrent caries. Itis an aim of many researchers in the dental area to develop compositematerials with higher strength, reduced shrinkage and higher resistanceto wear, which may be used in place of amalgam. Preferably the materialshould also be extrudable from a dental syringe since this procedure isnot only convenient and time saving for the dentist, but also helps toavoid the inclusion of air bubbles in the cavity.

[0003] The present invention provides composite materials with lowshrinkage and high surface hardness, and a method for preparing thesecomposite materials. Within certain ranges of the invention materialsare provided which may also be extruded from a dental syringe designedfor this use.

[0004] Composite materials for dental use and general methods for makingthem have been known for many years. See for example BP1401805, U.S.Pat. No. 3,740,850, U.S. Pat. No. 4,215,033, and U.S. Pat. No.3,801,344. These patents essentially describe mixtures of acrylateresins with glass or various metal oxides as filler and claim, forexample, the advantages of natural colour and desirable hardness.Although an improvement at the time, these early materials were stillrelatively week and had low resistance to wear. Improvements in surfacehardness were achieved when milling procedures improved and fillers withsmaller particle sizes became available. Attempts were also made to usedifferent types of monomer, for instance a silane containing monomer asin U.S. Pat. No. 4,504,231, or a mixture of monomers as in U.S. Pat. No.5,730,601. However the shrinkage of these materials still remained toohigh, at around 2 to 3 percent by volume. Further improvements were madeby the inclusion of two or more types of filler particle, for example aconventional glass fillers with particle size ranges of 0.5 to 40microns and 0.2 to 15 microns, together with a fine filler with particlesize in the range 5 to 150 nano metres, as in US4649165. These materialshowever still have the problem that it is hard to incorporate sufficientfiller into the composite to obtain the desired hardness. Attempts toovercome this problem have also included the use of surfactants, as inU.S. Pat. No. 4,374,937. However there remains the problem that use oftoo much conventional filler material in a composite leads to a stiffpaste which is hard to manipulate, and ultimately to a dry andnon-cohesive mixture. It is desirable to be able to extrude a dentalfilling material directly from a syringe into the tooth cavity, and thisis not possible with such stiff pastes. Such pastes when cured typicallyhave a yield strength around 130 MPa, a surface Vickers hardness ofabout 70, and a volume shrinkage between 2.5 to 3 percent. The yieldstrength is the maximum load that may be applied to a material beforepermanent deformation and damage occurs, and it is desirable that thisis a high as possible. A high surface hardness is needed because thisreduces abrasive wear of the material, while low shrinkage is desirablein order to minimise gap formation around a filling.

DETAILED DESCRIPTION OF THE INVENTION

[0005] In the present invention it has been found that by the use offiller combinations with a certain defined size distribution and sizerelationship, an unexpectedly high amount of filler may be combined withthe resin matrix without the paste becoming either stiff and hard tohandle, or crumbly and non-cohesive. The resulting pastes when curedhave a yield strength of at least 180 MPa, a Vickers hardness of about90, and a volume shrinkage on curing less than 2%. A conventionally madecomposite paste commonly contains ground glass particles with a particledistribution of about 0.05 to 1 micron together with small amounts of asiliceous filler with a particle size distribution between about 0.01and 0.1 microns. This latter is added to adjust the handling propertiesof the paste. The total amount of filler contained in such a compositionto give a consistency which is clinically useful is commonly around 75%,but may be as high as about 80% in some cases. Typical physicalproperties are a yield strength around 130 MPa, a Vickers hardnessaround 70, and shrinkage on cure of about 2.5 to 3.0%. The Shore Ahardness of the uncured paste is used as a measure of “packability” and“handling characteristics” of the paste, and it has been determinedempirically that for filling materials which are to be used in posteriorcavities an optimum value for the Shore A hardness is between about 50and 55. A paste made as above typically has a Shore A hardness of around30 to 45. In the present invention, a paste as above is taken and mixedwith an additional fraction of filler chosen such that the mean particlesize of this additional filler is at least about twenty times the meansize of the first filler in the paste, and is preferably essentiallymono-modal. The particle size distribution of a powder is convenientlyindicated by the span, which is defined as (D(v, 0.9)−D(v, 0.1))/D(v,0.5) where D(v, 0.5) is the mean particle size with 90% of the particlessmaller than D(v, 0.9) and 10% of the particles smaller than D(v, 0.1).In practice the particle size of the additional filler will have adistribution, but the span as defined above should be as small aspossible and should preferably be less than about 1.3. Thus in the aboveexample, the mean particle size of the glass (first filler) contained inthe paste is 0.8 micron, and the mean size of the additional fillershould therefore be at least about 16 microns, with no more than 10% ofthe particles either larger than 27 microns or smaller than 5 microns.However, the additional filler may also be larger, for example with amean size of about 65 microns. In this case the D(v, 0.9) should not belarger than 115 microns, and the D(v, 0.1) and the D(v, 0.1) should notbe smaller than 16 microns. Such a filler fraction may conveniently bemade by passing milled glass through commercially available sieves.

[0006] Therefore, according to the present invention an intra-oral orextra-oral dental restorative material includes at least onpolymerisable monomer, a first solid filler component with a meanparticle size between 0.1 and 5 microns, a second solid filler componentwith a mean particle size at least ten times greater than the meanparticle size of the first solid filler component and a particle sizespan of less than 1.5.

[0007] Another preferred embodiment of the invention includes a similarmaterial wherein the second solid filler component has a mean particlesize at least twenty times greater than the mean particle size of thefirst solid filler component. The particle size span of the secondfiller component may also be less than 1.3.

[0008] According to another embodiment of the invention, about 70% ofthe particles in the second filler component are within the range of 75to 125 percent of its mean size. In another embodiment, at least 80percent of the particles in the second filler component are within thatrange.

[0009] A preferred material also includes from about 5 to 25 percent byweight of the polymerisable component, or more preferably from about 10to 15 percent by weight of that component.

[0010] The Vickers hardness of such a cured material according to thepresent invention is preferably greater than 80 and the yield strengthis at least 180 MPa. The Vickers hardness may also be greater than 90.

[0011] One of the solid filler components may include a fluoridecontaining glass or spherical or essentially spherical particles orfluoride containing glass particles.

[0012] The material also may contain between 0 and about 5 percent of asolid filler having a mean particle size of from about 1 to 100 nm.

[0013] When the material according to the present invention is extrudedfrom a syringe or a dental syringe with a tip diameter of 2 mm, theextrusion force is preferably less than about 300 Newtons.

[0014] The following examples serve to illustrate the invention further,but are not restrictive in any way. Since the materials described in theexamples are sensitive to and hardened by exposure to light between 400and 500 nm, all preparations were done in yellow light devoid of lightin this wavelength range. In the following examples “parts” means “partsby weight”.

EXAMPLE 1

[0015] Preparation of the Initial Paste

[0016] A resin mixture was first made by combining urethane resin 44parts, TCB resin 34 parts, trimethylolpropane trimethacrylate 20 parts,camphor quinone 0.28 parts, dimethylaminobenzoic acid ethyl ester 0.59parts, butylated hydroxytoluene 0.1 parts, hydroquinone monomethylether0.025 parts, and 2-hydroxy-4-methoxybenzophenone 1.0 part in a flask andstirring at 50° C. until a clear homogenous mixture was obtained.

[0017] This resin (25.3 parts) was then mixed at 50° C. with 74.7 partsof a powder mixture comprising silanated strontium glass with a meanparticle size of 0.8 microns and a span as defined above of 1.6 94.5parts, strontium fluoride 5 parts, and hydrophobic fumed silica(particle size range about 0.05 microns) 0.5 parts, to give a stiffpaste after cooling. The overall filler content of this paste is 74.7%by weight. The properties of this paste were measured and results aregiven in Table 1

EXAMPLE 2

[0018] Preparation of a the Addition Filler with Narrow Particle SizeDistribution

[0019] Glass frit with a particle size of about 2 to 5 mm was milled ina dry ball mill to give a powder with a particle size ranging from about1 micron to 1 mm and a mean particle size of about 50 microns. This wassieved over a mesh with an aperture size of 250 microns, and the coarsefraction remaining in the sieve was discarded. The glass which passedthrough was sieved over a mesh with an aperture or 100 microns to obtaina fraction with a particle range between 100 and 250 microns and a spanof about 0.9. This is termed fraction

[0020] A. The remaining glass was sieved over a mesh with an aperture of85 microns, and the glass remaining in the sieve was discarded. Theglass which passed through was collected and sieved over a mesh with anaperture size of 48 microns, the fraction which passed through beingdiscarded. In this way a fraction with a particle size range of 48 to 85microns was obtained. This is termed fraction B. Both fractions A and Bwere separately silanated by slurrying them at room temperature with3-(trimethoxysilyl)propyl methacrylate (1% of the weight of the glass)dissolved in water acidified with acetic acid. After one hour the glassfractions were filtered off and dried for 18 hours at 85° C. in an oven.When the particle size of fraction B was measured using a MalverMastersizer S ver 2.10, the mean particle size was shown to be about 60microns, with a D(0.9) of 89.5 microns and a D(0.1) of 14.2 microns.

[0021] This corresponds to a span of 1.26

EXAMPLE 3

[0022] Preparation of a First Paste Containing Additional FillerFraction B

[0023] UK221581

[0024] Two hundred grams of the paste from example 1 was taken and mixedat 50° C. with 160 grams of silanated glass fraction B from example 2using a planetary mixer. After mixing for 15 minutes the paste hadformed a coherent mass. This was cut up and was spread around the mixingpot before being mixed for a further 15 minutes. The paste mass wasagain cut up, spread around the pot and mixed for 15 minutes, but thistime a vacuum of 220 mbar was applied for the last 10 minutes. Theresulting paste when cool was only marginally stiffer than the originalpaste from example 1 even though it contained a total of 86% filler byweight. The properties of the paste were measured and these are given inTable 1.

EXAMPLE 4

[0025] Preparation of a Second Paste Containing Additional FillerFraction B

[0026] UK221591

[0027] The paste from example 3 (342 grams) was taken and 13 grams ofsilanated glass from example 2 fraction B was added using the procedureas given in example 3. This paste contained a total of 86.5% filler. Theproperties were measured and results are given in Table 1.

EXAMPLE 5

[0028] Preparation of a Third Paste Containing Additional FillerFraction B

[0029] UK221592

[0030] Paste from example 4 (293 grams) was taken and 12 grams ofsilanated glass from example 2 fraction B was added using the procedureas given in example 3. This paste contained a total of 87.0% filler. Theproperties were measured and results are given in Table 1.

EXAMPLE 6

[0031] Preparation of a Fourth Paste Containing Additional FillerFraction B

[0032] UK221592

[0033] Paste from example 5 (265 grams) was taken and 10 grams ofsilanated glass from example 2 fraction B was mixed in using theprocedure as given in example 3. This paste contained a total of 87.5%filler. The properties were measured and results are given in Table 1.

EXAMPLE 7

[0034] Preparation of a Fifth Paste Containing Additional FillerFraction B

[0035] UK221593

[0036] Paste from example 6 (235 grams) was taken and 10 grams ofsilanated glass from example 2 fraction B was mixed in using theprocedure as given in example 3. This paste contained a total of 88.0%filler. The properties were measured and results are given in Table 1.

EXAMPLE 8

[0037] Preparation of a First Paste Containing Additional FillerFraction A

[0038] UK221511

[0039] One hundred grams of the paste from example 1 was taken and mixedat 50° C. with 158.3 grams of silanated glass fraction A from example 2using a planetary mixer. After mixing for 15 minutes the paste hadformed a coherent mass. This was cut up and was spread around the mixingpot before being mixed for a further 15 minutes. The paste mass wasagain cut up, spread around the pot and mixed for 15 minutes, but thistime a vacuum of 220 mbar was applied for the last 10 minutes. The pastecontained a total of 90% filler, but was judged to be too dry and stiff.A further 10 grams of the paste from example 1 was therefore added andthe mixing procedure outlined above repeated. The resulting pastecontained a total of 89.6% filler by weight. The properties of the pastewere measured and these are given in Table 1.

EXAMPLE 9

[0040] Preparation of a Second Paste Containing Additional FillerFraction A

[0041] UK221541

[0042] The paste from example 6 (233.7 grams) was taken and warmed to50° C. in a planetary mixer. Silanated fraction A from example 2 (10.31grams) was added and the mixing procedure carried out as outlined inexample 3. This paste contained a total of 89% filler. The properties ofthe paste were measured and these are given in Table 1.

EXAMPLE 10

[0043] Preparation of a Third Paste Containing Additional FillerFraction A

[0044] UK221542

[0045] The paste from example 7 (199 grams) was taken and warmed to 50°C. in a planetary mixer. Silanated fraction A from example 2 (14.96grams) was added and the mixing procedure carried out as outlined inexample 3. This paste contained a total of 88.6% filler. The propertiesof the paste were measured and these are given in Table 1.

EXAMPLE 11

[0046] Preparation of a Paste Containing an Additional Filler Fractionwith Spherical Particles

[0047] UK221671

[0048] Spherical glass beads with a particle size ranging from 40 to 70microns and a particle size span of about 0.5 were silanated byslurrying them at room temperature with a solution of3-(trimethoxysilyl)propyl methacrylate (1% of the weight of the glass)in water acidified with acetic acid. After one hour the glass sphereswere filtered off and dried for 36 hours at 85° C. in an oven. Twohundred grams of the paste from example 1 were mixed with two hundredand twenty two grams of these silanated spheres following the procedurein example 3. The resulting paste felt only marginally stiffer than thepaste from example 3 even though it contained an extremely high totalsolid filler content of 88%. The properties were measured and theresults are given in Table 1. It is notable that the Shore A hardnessvalue of this paste was only 3 units higher than that of the startingpaste, even though the solid filler content of the two pastes differedby 13%.

EXAMPLE 12

[0049] Preparation of a Second Paste Containing an Additional FillerFraction with Spherical Particles

[0050] UK221681

[0051] The paste from example 11 (382 grams) was mixed as described inexample 3 with further silanated glass spheres (34.4 grams). The resultwas a stiff paste which was nevertheless easily spatulated andclinically adaptable to a tooth surface. The properties were measuredand the results are given in Table 1.

EXAMPLE 13

[0052] Preparation of a Second Paste Containing an Additional FillerFraction with Both Spherical and Irregular Shaped Particles

[0053] UK221722

[0054] The filler fraction B was mixed with an equal weight of thespherical filler as used in example. A paste was produced containing88.5% total weight percent filler, using the method as in example 3. Theproperties are given in Table 1.

EXAMPLE 14

[0055] Preparation of a Third Paste with an Additional FractionComprising Spherical Particles with a Narrow Particle Size Distribution

[0056] Spherical glass beads with a mean size of 25.2 microns, a D(v,0.9) of 36.2 microns, and a D(v, 0.1) of 16.8 microns (span=0.8) weresilanated as in example 11. These spheres were mixed in a planetarymixer with the resin mixture as in example 1 (13.2 parts), glass powderwith a mean particle size of 0.8 microns (36.8 parts) and strontiumfluoride with a mean particle size of 0.8 microns (2.7 parts) until ahomgenous paste was obtained. The properties were measured and are givenin the table.

EXAMPLE 15

[0057] Preparation of a Fourth Paste with an Additional FractionComprising Spherical Particles with a Narrow Particle Size Distribution

[0058] Spherical glass beads with a mean size of 17.7 microns, a D(v,0.9) of 29.6 microns, and a D(v, 0.1) of 10.4 microns (span=1.08) weresilanated as in example 11. These spheres were mixed in a planetarymixer with the resin mixture as in example 1 (13.6 parts), and glasspowder with a mean particle size of 0.8 microns (37.95 parts) until ahomgenous paste was obtained. The properties were measured and are givenin the table.

COMPARATIVE EXAMPLE 1 Using Additional Filler with a Wide Particle SizeDistribution

[0059] UK221631

[0060] The glass powder used in example 2 was taken and passed through a250 micron sieve to remove coarse particles. When the particle size wasmeasured using a Malvern Mastersizer S 2.10 the mean size was found tobe 39.3 microns with a D(0.9) of 132.4 and a D(0.1) of 2.0 microns,corresponding to a span of 3.3. This was silanated as described inexample 2. The silanated glass powder (100 grams) was added to the pastefrom example 1 (200 grams) using the mixing procedure describedpreviously in example 3. The resulting paste contained a total of 83.1%filler and was a very stiff paste. A further 10 grams of the silanatedglass powder above was added and the mixing procedure repeated. Theresulting paste contained a total of 83.7% filler and was dry, crumblyand only just coherent. It was not possible to add more glass and stillobtain a cohesive paste. The properties were measured and results aregiven in Table 1. This paste was very stiff and could not be extrudedfrom a dental syringe.

[0061] Measurement of Extrusion Force

[0062] Dental syringes (Hawe-Neos Dental, CH-6934 Bioggio, Switzerland,article number 436) with an internal tip diameter of 2 mm were filledwith the material to be tested. After allowing the syringe and contentsto equilibrate to 23° C., the syringes were mounted in a universaltesting machine (Zwick) and the material extruded by pushing the pistoninto the syringe at a constant velocity of 28 mm per minute. The forceon the piston, or extrusion force, was noted. At least threemeasurements for each material were taken and the average extrusionforce was calculated. Further syringes were tested by hand, and theability to extrude the paste from the syringe judged from the hand forceneeded.

[0063] Measurement of Yield Strength

[0064] Metal forms with an internal diameter of 4 mm and a height of 6mm as described in ISO 9917 section 7.4 were used to prepare thespecimens. The paste to be measured was filled into the forms, coveredwith polyester foil, and pressed with metal plates to extrude excessmaterial. The material was then cured for 40 seconds from each end usinga dental curing lamp (Spectrum Lite, Dentsply) with an output between600 and 700 mW/cm². The forms complete with specimen were drawn acrosssilicon carbide paper (600 grit) until a smooth surface level with theend of the form was obtained, and then the cured specimens were removedfrom the form. The specimens were stored in water at 37° C. for 24 hoursbefore being tested in a universal testing machine (Zwick) with acrosshead speed of 1 mm/minute. The stress strain curve for eachspecimen was inspected and found to consist essentially of an initialstraight portion followed by a curved portion leading to the finalbreaking point. The straight portion of the curve corresponds to elasticbehaviour of the material, whereas the curved portion corresponds toplastic flow. The force at which the stress strain curve first deviatedfrom a straight line was taken as the yield point. The yield point isexpressed in MPa, and is calculated by dividing the yield force inNewtons by the cross-sectional area of the specimen. The average valueof at least five specimens for each material was calculated.

[0065] Properties of the Pastes TABLE 1 Additional Yield Vickers fillerExtrusion Material from % str. hardness Shrinkage D(v, 0.5) forceExtrusion example filler MPa HV5 volume % and span Newtons by hand  174.7 154.4 63.7 2.6 none 92 Easily extrudable  3 (UK221581) 86.0 200.078.9 1.6   60, 1.26 Extrudable  4 (UK221591) 86.5 200.0 87.2   60, 1.26Extrudable  5 (UK221592) 87.0 203.9 95.6 1.5   60, 1.26 290 Extrudable 6 (UK221593) 87.5 205.2 95.7   60, 1.26 Extrudable but stiff  7(UK221611) 88.0 215.6 94.5 1.4   60, 1.26 Extrudable but stiff  8(UK221511) 89.6 198.0 77.0 1.1  175, 0.9 Not extrudable  9 (UK221541)89.0 197.0 68.6  175, 0.9 Not extrudable 10 88.6 195.0 67.9 1.3  175,0.9 Just (UK221542) extrudable 11 88.0 188.4 71.5  175, 0.9 120 Easily(UK221671) extrudable 12 89.0 191.1 62.4  175, 0.9 Easily (UK221681)extrudable 13 88.5 192.3 1.2  175, 0.9 Easily (UK221722) extrudable 14(UK25-61-1 86.8 187 87.0 1.5 25.2, 0.8 130 Easily extrudable 15(GB8-114- 86.7 223 91.0 1.6 17.7, 1.08 65 easily 1) extrudableComparative 83.7 205.0 68.3 1.8 39.3, 3.3 >500 Not example 1 extrudable(UK221631)

[0066] From Table 1 it can be seen that the conventional paste ofexample 1 has a yield strength of only 154 MPa, a Vickers hardness of63.7, a volume shrinkage of 2.6%, and an extrusion force from a dentalsyringe of 92 Newtons. This paste may therefore be easily extruded froma dental syringe, but has an unacceptably high shrinkage as well as lowsurface hardness and low yield strength.

[0067] When the filler loading is increased by adding additional fillerwith a conventional broad particle size distribution with a span of 3.3as in comparative example 1, the yield strength is improved and theshrinkage is reduced below 2%, but the surface hardness is barelychanged and the paste becomes so stiff that it can no longer be extrudedfrom a dental syringe. In contrast the formulations from examples 3, 4,5, 6, 7, 14, and 15 which fall within the scope of the present inventionhave improved yield strengths, improved surface hardness, reduced volumeshrinkage, and are also extrudable from a dental syringe.

[0068] The present invention therefore allows the formulation of pasteswith required optimum properties of yield strength, surface hardness andextrudability from a dental syringe, which are not otherwise possible.Different methods of particle size measurement give different results,and the values given for purposes of the present invention aredetermined by sieving or by use of a malvern mastersizer s version 2,10laser diffraction particle size analyser

What is claimed is:
 1. An intra-oral or extra-oral dental restorativematerial comprising: a) at least one polymerisable monomer b) a firstsolid filler component with a mean particle size between 0.1 and 5microns c) a second solid filler component with a mean particle size atleast ten times greater than the mean particle size of the first solidfiller component and a particle size span of less than 1.5.
 2. Anintra-oral or extra-oral dental restorative material comprising: a) atleast one polymerisable monomer b) a first solid filler component with amean particle size between 0.1 and 5 microns c) a second solid fillercomponent with a mean particle size at least twenty times greater thanthe mean particle size of the first solid filler component and aparticle size span of less than 1.5.
 3. A material as in claim 1 inwhich the particle size span of the second filler is less than 1.3.
 4. Amaterial as in claim 2 in which the particle size span of the secondfiller is less than 1.3.
 5. A material as in claim one in which 70percent of the particles in the second filler component are within therange of 75 to 125 percent of its mean size.
 6. A material as in claimone in which 80 percent of the particles in the second filler componentare within the range of 75 to 125 percent of its mean size.